Because seemingly only in the U.S. the legislature hasn’t meaningfully passed significant new legislation, making the immigration law out of date. A lot of intelligent people voted for Obama, and Obama created DACA, which says overstaying your visa as a child is fine and results in “deferred” deportation. Hence a lot of leftists think visa rules in the U.S. are so outdated that violating them is fine. IMO Obama made a careful calculation to degrade the sanctity of immigration laws to avoid hardship on a million people; it was a good idea on humanitarian grounds but it did open a large can of worms.

As for why ICE are the bad guys, well that’s a completely different question. The way they enforce laws is thug-like and have resulted in multiple innocent deaths. Regardless of if it is ICE or a normal police department if enforcing laws causes death you bet there will be a large pushback.